This invention relates to the field of micro-electro-mechanical device fabrication, more particularly to the fabrication of micromirror devices.
Micromechanical devices are small structures typically fabricated on a semiconductor wafer using techniques such as optical lithography, doping, metal sputtering, oxide deposition, and plasma etching that have been developed for the fabrication of integrated circuits.
Micromirror devices are a type of micro-electro-mechanical systems (MEMS). Other types of MEMS devices include accelerometers, pressure and flow sensors, gears and motors. While some micromechanical devices, such as pressure sensors, flow sensors, and micromirrors have found commercial success, other types have not yet been commercially viable.
Micromirror devices primarily are used in optical display systems. In display systems, the micromirror is a light modulator that uses digital image data to modulate a beam of light by selectively reflecting portions of the beam of light to a display screen. While analog modes of operation are possible, micromirrors typically operate in a digital bistable mode of operation and as such are the core of the first true digital full-color image projection systems.
Micromirrors have evolved rapidly over the past ten to fifteen years. Early devices used a deformable reflective membrane which, when electrostatically attracted to an underlying address electrode, dimpled toward the address electrode. Schlieren optics illuminate the membrane and create an image from the light scattered by the dimpled portions of the membrane. Schlieren systems enabled the membrane devices to form images, but the images formed were very dim and had low contrast ratios, making them unsuitable for most image display applications.
Later micromirror devices used flaps or diving board-shaped cantilever beams of silicon or aluminum, coupled with dark-field optics to create images having improved contrast ratios. Flap and cantilever beam devices typically used a single metal layer to form the top reflective layer of the device. This single metal layer tended to deform over a large region, however, which scattered light impinging on the deformed portion. Torsion beam devices use a thin metal layer to form a torsion beam, which is referred to as a hinge, and a thicker metal layer to form a rigid member, or beam, typically having a mirror-like surface: concentrating the deformation on a relatively small portion of the micromirror surface. The rigid mirror remains flat while the hinges deform, minimizing the amount of light scattered by the device and improving the contrast ratio of the device.
Recent micromirror configurations, called hidden-hinge designs, further improve the image contrast ratio by fabricating the mirror on a pedestal above the torsion beams. The elevated mirror covers the torsion beams, torsion beam supports, and a rigid yoke connecting the torsion beams and mirror support, further improving the contrast ratio of images produced by the device.
Throughout the development of micromirror devices, as with the development of semiconductors, great strides have been made to improve the fabrication yield of the manufacturing process. Blocked etch at the mirror level is one of the major causes of loss during the micromirror fabrication process. A new process is needed to help prevent blocked mirror etch without introducing other problems in the micromirror fabrication flow.
Objects and advantages will be obvious, and will in part appear hereinafter and will be accomplished by the present invention which provides a method for patterning a metal layer and a cleanup process that removes metal etch residues but does not harm remaining photoresist. One embodiment of the disclosed invention provides a method of fabricating a semiconductor device. The method comprises: depositing a first photoresist layer on a wafer of partially formed devices, depositing a metal layer over the first photoresist layer, depositing a second photoresist layer over the metal layer, patterning the second photoresist layer to expose regions of the metal layer, etching the metal layer to remove the exposed regions of the metal layer, and cleaning residue created by the etching using a photoresist developer. The cleaning does not harm the first and second photoresist layers which remain after the cleaning.
Another embodiment of the disclosed invention provides a method of patterning a metal layer. The method comprises: depositing a metal layer on a wafer of partially formed devices, depositing a photoresist layer over the metal layer, patterning the photoresist layer to expose regions of the metal layer, etching the metal layer to remove the exposed regions of the metal layer, and cleaning residue created by the etching using a photoresist developer. The cleaning process does not remove the patterned photoresist layer.
According to one embodiment of the disclosed invention, the patterning of the photoresist layer defines mirrors in the metal layer. According to another embodiment of the disclosed invention, the patterning of the photoresist layer defines electrical interconnections in the metal layer.
According to an alternate embodiment of the disclosed invention, which is not used in the manufacture of micromirror devices but may be useful to manufacture isolated or insulated interconnections in other semiconductor devices, the remaining photoresist is reflowed, typically by heating the remaining photoresist. Another metal layer may be deposited on the reflowed photoresist layer and patterned. Additional layers of reflowed photoresist and patterned metal may be fabricated. The reflowed photoresist layers may be removed, leaving an insulating air gap between the patterned metal layers. This air gap may later be filled with an insulator.